1. Field of the Invention
The present invention relates to a screw vacuum pump, and more particulary a positive displacement screw vacuum pump which is designed to have a single stage screw rotor and to reduce the power consumed in a high vacuum range.
2. Description of the Prior Art
Vacuum pumps are widely used in various industries, such as semiconductor manufacturing industry, metallurgical industry, chemical industry, and the like.
As a known vacuum pump, there exists a water sealed vacuum pump, a Root's type vacuum pump, a screw type vacuum pump and an ejector type vacuum pump, for example.
In the water sealed vacuum pump, foreign matter is led from a suction opening to a discharge opening under the condition where it directly contacts with water during obtaining vacuum. Therefore, the water sealed vacuum pump cannot successfuly be used in the refining industries such as, semiconductor manufacturing industry, pharmaceutical products industry and the like, which cannot have the ingression of impurities. Accordingly, a dry type or water free type vacuum pump has been used to ensure that the gas to be vacuumized is not in contact with the water.
However, although this type of vacuum pump is employable in a medium vacuum range, it is not suitable for use in a low vacuum range (less than 400 Tort) because the leakage of gas from between the rotors increases to remarkably raise the gas temperature, which results in burning of the rotors.
To solve the drawback of the water free type vacuum pump, a multi-stage screw vacuum pump has been suggested to avoid generation of heat and thus burning of the rotors. Although this multi-stage screw vacuum pump is suitable fox use over a low vacuum range to a high vacuum range, it has some disadvantages that the device is not simplified, the cost increased and the space required for a given pump capacity is increased.
Referring to FIG. 6 which shows a conventional Root's type multi-stage vacuum pump. The pump housing has formed therein three chambers seperated by partitions. A pair of shafts 20 within the chambers are mounted thereon three rotors, i.e., a first stage rotor 21, a second stage rotor 22, and a third stage rotor 23 respectively. These rotors have widths which is decreased with a geometrical ratio. The pump housing has formed therein a first stage inlet port 24, a second stage inlet port 26 and a third stage inlet port 28 at one side of the housing. On the opposite side of the housing, a first stage outlet port 25, a second stage outlet port 27 and a third stage outlet port 29, each communicating with the corresponding inlet port. The assembly is simplified and the space requirement has been reduced by using the unified screw rotors.
One disadvantage encountered with the Root's type multi-stage vacuum pump, howerever, is that they tend to experience significant reduction in pumping efficiency. For this reason, the use of the Root's type multi-stage vacuum pump is greatly limited.
There exists, therefore, a significant need for an improved vacuum pump capable of providing an efficient pumping performance at a relatively high pressure.
FIG. 7 shows multi-stage screw type vacuum pump, Japanese Patent Laid-Open 63-36086, which has been proposed to meet the above mentioned demands. The casing includes a rotor chamber having first suction opening 37 and first discharge opening 38 (encircled by alternate long and two short dashed lines respectively) and second suction opening 39 and second discharge opening 40 (encircled by broken line respectively), and a first pair of male 32 and female screw rotor 33 meshing with each other which are rotatably received in the rotor chamber, a second pair of male 34 and female screw rotor 35, the pitch P.sub.2 of these screw rotors being shorter than the pitch P.sub.1 of the first pair of screw rotors 32 and 33. All of the threaded portions of the screw rotors have a shape of an arc 50, Archimedean curve 51 and epitrochoid 52.
However, the screws of said Japanese Patent Laid-Open 63-36086 have a constant pitch such that there is no tendency to compress the gas along the length of the screw and therefore it is unsuitable for applying it in a relatively high vacuum range. Moreover, the pump has double stage screw rotors so that the assembly is complicated, space requirements increased, and the cost increased.
Thus, a single stage oil free type vacuum pump suitable for use over a low vacuum range to a high vacuum range has been required.
The present invention fulfills these needs and provides further related advantages.